Fluid swivels are commonly used in offshore installations to transfer fluids such as gas and oil between a fixed underwater pipeline, or underwater hydrocarbon well and a tanker that may be moored to a buoy around which it drifts under the influence of currents, winds, and waves. The fluid swivel, which may be located on a buoy to which the tanker is moored, may have to withstand high pressures, as where an undersea well produces hydrocarbons at high pressures. A typical multi-product fluid swivel includes a ring-shaped outer wall that rotates about a relatively fixed inner wall, with the walls forming a chamber between them through which fluid passes. There is a gap between the inner and outer walls leading in opposite directions from the chamber, and the gap must be sealed. A fluid swivel with a diameter at the middle of the chamber of at least about four feet is required, to provide room within the chamber for pipes leading to the inner wall and to facilitate rotation, where large volume flow rates are required.
Prior art swivels have used radial seals to seal the gap between the inner and outer walls, each seal having one side pressing radially outwardly against the outer wall and another side pressing radially inwardly against the inner wall. The life of the seal depends upon changes in the width of the gap across which it seals. When high pressure fluid enters the chamber, the large radial forces on the outer and inner walls cause the outer wall to expand and the inner wall to contract radially. As a result, the width of the gap across which the seal extends increases when the high pressure is applied. For very high pressures, the inner and outer walls must be made massive to avoid large increases in the width of the gap. Despite demand for reliable fluid swivels of at least about four feet diameter (for high volume flow) and capable of passing fluids at over 2000 psi, no manufacturer known to applicant has been able to supply such fluid swivels.
At least two seals are generally used along each side of the gap extending from the fluid-carrying chamber to the environment. One seal serves as the primary seal which generally withstands the pressure, while the other seal is a secondary or back-up seal which prevents leakage of hydrocarbons into the environment in the event of failure of the primary seal. However, if a primary seal on one side of the fluid-carrying chamber should fail, while the primary seal on the other side of the chamber remains intact, then unbalanced pressures could occur, such unbalanced loads could impose large axial loads on the bearings which rotatably couple the inner and outer walls of the swivel, and consequently increase the turning torque of the structure.
The "fatigue life" of a seal is inversely proportional to an exponential function of the width of the extrusion gap and the pressure across the seal. As a result, at very high pressures the life of the seals may be very short. A fluid swivel which has a lighter weight and whose seals had an increase life, especially in the case of swivels carrying high pressures, and which avoided axial unbalances in the event of failure of a primary seal, would be of considerable value.